1. Field of the Invention
This invention relates to infrared detectors, specifically, this invention relates to thin film infrared detectors that operate at room temperature.
2. Description of the Background:
A radiation detector is a device that produces an output signal which is a function of the amount of radiation that is incident upon an active region of the detector. Infrared detectors are those detectors which are sensitive to radiation in the infrared region of the electromagnetic spectrum. Infrared detectors include two types of detectors, thermal detectors and photon detectors.
Photon detectors function based upon the number of photons that are incident upon a transducer region of the detector. Photon detectors have a direct interaction between electrons and photons, are relatively sensitive and have a high response speed compared to thermal detectors. However, photon detectors operate well only at low temperatures and therefore require refrigeration to provide sensitive detection.
Thermal detectors function based upon a change in the temperature of the transducer region of the detector due to absorption of the radiation to be detected. Thermal detectors provide an output signal that is proportional to the temperature of the transducer region. Since radiation absorption usually occurs over a wide range of wavelengths, thermal detectors are typically responsive over a wide range of wavelengths. However, thermal detectors typically have a lower sensitivity and a slower response speed than photon detectors.
A bolometer is a thermal detector having a transducer region whose resistance depends upon its temperature. The voltage responsivity of a bolometer is a measure of the effectiveness of the bolometer at detecting radiation. The voltage responsivity of a bolometer is defined as follows: EQU R.sub.VB =dR/dT .eta.I.sub.b /(G(1-.omega..sup.2 .tau..sup.1).sup.1/2)
where I.sub.b is bias current that is passed through the transducer region of the detector, R is electrical resistance of the transducer region of the detector, .eta. is absorptivity of electromagnetic radiation incident upon a surface of the transducer region of the detector, G is the coefficient of thermal conductance of heat away from the transducer region of the detector, .omega. is angular modulation frequency of electromagnetic radiation incident upon the transducer region of the detector, T is the temperature of the transducer region, and .tau. is the thermal time coefficient of the transducer region of the detector. .tau. is equal to C/G where C is the heat capacity of the transducer region of the detector.
Normalized voltage detectivity D.sup.*.sub.VB for a bolometer is another measure of the sensitivity and is defined by EQU D.sup.*.sub.VB =(R.sub.VP (.DELTA.f.multidot.A).sup.1/2)/V.sub.n
where .DELTA.f is the frequency bandwidth (usually of an amplifier) associated with the bolometer and V.sub.n is the noise voltage of the output signal of the bolometer. High detectivity therefore requires (1) a low noise voltage V.sub.n and (2) a high responsivity R.sub.VP.
A pyroelectric detector is a thermal detector incorporating a pyroelectric material as the transducer material. Pyroelectric materials have an electric polarization and thereby a dielectric constant which are functions of temperature. As the temperature of the pyroelectric material changes, the electric polarization of the pyroelectric material changes. Insulating pyroelectric materials generate a surface charge that is proportional to their electric polarization because of the pyroelectric effect. A pyroelectric detector may be formed from a capacitor which has a pyroelectric material as its dielectric.
The responsivity of a pyroelectric detector R.sub.VP is defined as the ratio between the output voltage of the pyroelectric detector and the radiant power that is incident upon the pyroelectric detector. The normalized voltage detectivity of pyroelectric detector D.sup.*.sub.VP is defined as: EQU D.sup.*.sub.VP =(R.sub.VP (.DELTA.f.multidot.A).sup.1/2)/Vn
where R.sub.VP is the voltage responsivity of the pyroelectric detector, V.sub.n is the noise Voltage of the pyroelectric detector, .DELTA.f is the frequency bandwidth (usually of an amplifier) that is associated with the pyroelectric detector, and A is the area of a surface of the pyroelectric material which is heated by the incident radiant power.
Another measure of the sensitivity of a pyroelectric detector is the pyroelectric figure of merit M.sub.r which is defined as follows: EQU M.sub.r =p/(.rho.c.sub.p (.epsilon..sub.r Tan(.delta.)).sup.1/2)
where c.sub.p is the specific heat of the heated portion of the pyroelectric detector, .rho. is the density of heated portion of the pyroelectric detector, .epsilon..sub.r is the dielectric constant of the pyroelectric material (where .epsilon.=.epsilon..sub.r .times..epsilon..sub.0 and .epsilon..sub.0 is the permittivity of free space), and .delta.=(.sigma./(.omega..multidot..epsilon.)).sup.1/2, where .sigma. is the conductivity of the pyroelectric material and .omega. is the angular frequency at which incident radiation falling upon the pyroelectric detector is modulated. The change in permittivity and electric polarization of a pyroelectric material or layer provide measures of the change in temperature of the pyroelectric material.
The change in electric polarization of a pyroelectric material provides a pyroelectric current which is defined as the change in surface charge on the surface of the pyroelectric material per unit time that is generated by the change in magnitude of the electric polarization of the pyroelectric layer.
Prior art thermal detectors (i.e., bolometers and pyroelectric detectors) have been characterized by a D.sup.* for infrared detection of less than 10.sup.10. There is a continuing need for higher sensitivity, lower noise, and therefore higher detectivity thermal detectors.
Many of the prior art infrared detector materials use a transducing material for which there is no suitable thin film deposition technology. For example, barium strontium titanate transducers, which have been used as pyroelectric transducers, have been prepared by first forming bulk ceramics and then mechanically thinning the bulk ceramics in order to reduce their heat capacity.
It is desirable to have a transducer material that is compatible with thin film deposition and processing technologies. In addition, it is desirable to have a transducer material which can be vacuum deposited as a thin layer or film (i.e., having a thickness of less than a few microns) without requiring significant heating of the substrate.
Another technology which is unrelated to sensors per se, but which is coincidentally addressed by the present invention, is gate-insulated transistor technology. Transistors operate by switching a semiconductor conductive channel between a conducting "open" state and a non-conducting "closed" state. Gate-insulated transistors use the voltage applied from a gate electrode to affect the potential of a conductive channel in the semiconductor. The potential applied to the conductive channel by the gate electrode determines whether charge carriers (i.e., electrons or holes) travel along the conductive channel when an electromotive force exists along the conductive channel. Thus, the voltage of the gate electrode determines whether the conductive channel is in the conducting "open" state or the nonconducting "closed" state.
Typically, the voltage applied to the gate electrode must be continuously maintained in order to maintain a potential at the conductive channel and the particular switching state of the transistor (i.e., either the "open" state or the "closed" state). An electrically insulating layer, called a gate insulator layer, separates the gate electrode from the conductive channel of the transistor.
When the gate insulating material includes a layer of ferroelectric material, the polarization of the ferroelectric layer affects the potential of the conductive channel. Therefore, it is possible to switch the conductive state of the channel of the transistor between the "open" state and the "closed" state, by inverting the polarization of the layer of ferroelectric material.
The polarization of the layer of ferroelectric material may be inverted by applying a voltage pulse of suitable polarity to the gate electrode to generate an electric field at the ferroelectric layer that will invert the polarization of the ferroelectric layer. When a voltage pulse is applied to the gate electrode which switches the ferroelectric state of the ferroelectric gate insulator, there is a change in the electric polarization of the ferroelectric gate. This electric polarization change results in a change in the potential of the conductive channel of the transistor.
Gate-insulated transistors including a ferroelectric insulator layer in the gate insulator are also desirable for memory cells. A ferroelectric layer in the gate insulator attracts or repels charge carriers in the conductive channel of the transistor thereby providing a mechanism to store charge. The capacity of charge storage of a gate-insulated transistor incorporating a ferroelectric insulator layer in the gate insulator is greater than the capacity of a gate-insulated transistor without a ferroelectric layer in the gate insulator.
However, the materials that are known to be ferroelectric and which can be deposited in a layer on conventional semiconductors such as silicon must be deposited at relatively high deposition temperatures. These high deposition temperatures are detrimental to the structures that are necessary to form devices in conventional semiconductor substrates, such as silicon substrates. Thus, there is a long felt need for a ferroelectric material which is compatible with conventional semiconductor processing technology so that it can be used as a ferroelectric layer in a gate insulator for silicon based transistors.